Our overall objective is to learn more about the molecular, cellular and genetic basis for cellular senescence. In particular, we will study the cessation of proliferation in normal human fibroblast-like cells in vitro. The first part of our proposal addresses the question of how cessation of DNA synthesis is regulated in senescent human cells. We propose to identify and characterize some of the factors involved in DNA synthesis regulation in senescent and nonsenescent human cells, primarily through a series of somatic cell fusion experiments. In this way, we hope to learn how cessation of DNA synthesis is regulated in senescent cells and how nonsenescent cells escape this regulation. The second part of our proposal is to determine whether cessation of DNA synthesis associated with senescence, and cessation of DNA synthesis associated with terminal differentiation, are controlled in a similar manner. If these experiments show that senescence in human fibroblast-like cells is analogous to terminal differentiation in other kinds of cells, this will support the hypothesis that cellular senescence is governed by a specific genetic program, rather than by an accumulation of random events.